Conventional ink-jet printing can employ thermal ink jet printheads, piezoelectric printheads, or electrostatic printheads. In an electrostatic printhead, droplets of ink are selectively ejected from a plurality of drop ejectors in the printhead. Each of the plurality of drop ejectors includes a flexible sealed chamber with air on the inside and ink on the outside. The chamber containing the ink, located above the flexible chamber, has an inlet connected to an ink reservoir and a nozzle to eject the ink. Upon application of a voltage to an electrode inside the air chamber, the grounded membrane ceiling is deflected downward, thereby increasing the volume of the ink chamber and lowering its pressure. This causes ink to flow into the ink chamber from the ink reservoir. Then the electrode is grounded and the membrane pops up, creating a pressure spike in the ink cavity that ejects a drop from the nozzle. When making these sealed chambers, there needs to be a way to get an electrical trace from the electrode inside the chamber to a bond pad on the outside without damaging the integrity of the seal and without shorting the signal to ground. Furthermore, whatever strategy is used must withstand a long hydrofluoric acid (HF) etch which is used to etch away the silicon dioxide from inside the devices. The actual etching of the oxide inside the eventual air cavity is done through a series of holes in the ceiling that are later plugged by oxide and metal, but there is no way for the trace to exit vertically through these same holes.
Conventionally, the trace is passed through a long tube, which is long enough so that it is never completely released, and the remaining oxide acts as a plug to maintain the integrity of the seal. However, the partially-released tube is a liability because later wet processing can cause contamination to get inside. Removal of the contamination is very difficult because of the length of the tube, its small cross-section, and the sealed end. This contamination can cause shorting between the trace and the grounded plate above. Additionally, the long tube takes up significant chip area, adding more than 10% to the die area despite being folded up for compactness. Since the 10% is all in the process direction, the “waterfront” of the chip is also 10% bigger. Waterfront is the amount of space occupied by the printhead face or die length in the process direction. In a drum architecture, this is in the direction of the circumference of the drum. Having a large waterfront adversely affects image quality due to the larger spread of distances between the planar printhead and the cylindrical intermediate drum. Large waterfront also consumes large space within the printer, necessitating larger drum diameter or splitting the head into smaller “facets”.
Accordingly, there is a need for new ways to get an electrical trace from the electrode inside the chamber to a bond pad on the outside.